1. Field of the Invention
The present invention relates to a vacuum interrupter, and more specifically to an electrode for the vacuum interrupter, which serves to prevent generation of a switching surge caused by a multi-reignition and a three-phase simultaneous breaking associated with the multi-reignition. (It will, hereinafter, be abbreviated into the three-phase simultaneous breaking.).
2. Description of the Prior Art
Generally, an electrode of a vacuum interrupter (Patent Specifications, for examples, U.S. Pat. Nos. 3,818,163 and 3,960,554) should meet the following conditions;
(1) to have a high static withstanding voltage PA1 (2) to have a large electric current breaking capacity PA1 (3) to have a large electric current flowing capacity PA1 (4) to be easily separable without welding together of the contact surfaces PA1 (5) to withstand the overvoltage caused in the switching surge, and PA1 (6) to have a long electric and mechanical endurance.
Surges by the current chopping and the reignition have been known. However, a research of a switching surge mechanism has been recently developed, which has revealed the fact that the switching surge includes surges caused by multi-reignition and three-phase simultaneous breaking.
Surging caused by multi-reignition is a phenomenon in which ignitions and extinctions of arc are alternated as a result of the competition between the interelectrode dielectric strength recovered by the current breaking operation of a vacuum interrupter and the recovery voltage appearing between the interelectrode immediately after the current breaking, and thus, the interelectrode voltage increases gradually with time.
Such surging is caused in the following cases; (1) high-frequency arc extinction in which the high-frequency (commercial frequency to 1000 kHz, for example 200 kHz) current flowing through the electric circuit is broken at its zero point, (2) in which the current chopping happens in an insufficient arcing time, (3) in which after the contacts are separated prior to a current zero point of commercial frequency current, the arc extinction takes place immediately nearest a current zero point.
The surge by the three-phase simultaneous breaking is a phenomenon in which the multi-reignition caused in one of the three phases of the commercial frequency current causes high-frequency current flowing through the inter-phase impedance to the other two phases, so that the high-frequency current offsets the commercial frequency currnt of the other two phases, consequently a current zero point occurs at least one of the two phases and/or current zero points occur at the two phases, where the three-phase commercial frequency current is interrupted simultaneously at the three phases thereof.
Also, this surge is extremely large as well as that caused by the chopping current larger than the chopping current of the vacuum interrupter, or commercial frequency current at their crest value.
Since electrodes of a vacuum interrupter which are made of metallic contact materials for vacuum interrupters commercially applied at present have, in themselves, no capacity to protect the electric circuit from surges by the multi-reignition and the three-phase simultaneous breaking, a surge suppressor or absorber is provided for protecting the electric circuit within a switchgear comprising a vacuum interrupter.
Therefore, the switchgear of the prior art is large in the size thereof, reliability for protecting the electric circuit of the electric apparatus provided with the switchgear is low, and its manufacturing cost increases. In order to solve such problems, an electrode proper of a vacuum interrupter is desired to prevent surge by the multi-reignition and the three-phase simultaneous breaking.